A magical effect caused by "True Magick", the Enlightened effect of will on reality, that fits within the local Consensus and could be dismissed as coincidence. Coincidental magic does not incur paradox.
For Example: A ricochet off of a nearby pipe happens to hit the target, A jet of flame shooting out from a nearby pipe, an old man has a heart attack. Each of these effects could be coincidental, or could be the work of a will worker. By design and definition, coincidental magic can't be distinguished from coincidence.
Vulgar magic, by contrast, is obviously not coincidence. Stopping a bullet in mid air, hurling a fireball from an open palm, or draining the life from a healthy woman across the room are all vulgar effects.
Vulgar and coincidental are subjective, and often depend on the perspective of the consensus of the local area. a Verbena's health potion might be incredulous in a modern western city, but might go uncontested in a rural eastern European farming village, and could even be mundane in a Verbena shard realm.